Calculator
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Formula Used
Diameter to radius: r = d ÷ 2
Radius to diameter: d = r × 2
Circumference: C = 2πr or C = πd
Circle area: A = πr²
The radius measures from a circle’s center to its edge. The diameter crosses the entire circle through its center. Therefore, every diameter always equals two matching radii.
To find radius, divide the known diameter by two. To reverse the process, multiply the known radius by two. The selected units remain consistent during direct calculations.
How to Use This Calculator
- Select diameter-to-radius or radius-to-diameter mode.
- Enter one value or paste several batch values.
- Select the input unit and desired output unit.
- Choose decimal precision and number notation.
- Press Calculate to view detailed circle measurements.
- Copy, print, or download the displayed results.
Worked Examples
| Known Value | Calculation | Answer |
|---|---|---|
| Diameter = 10 cm | 10 ÷ 2 | Radius = 5 cm |
| Diameter = 25.4 mm | 25.4 ÷ 2 | Radius = 12.7 mm |
| Radius = 6 in | 6 × 2 | Diameter = 12 in |
| Diameter = 2 m | 2 ÷ 2 | Radius = 1 m |
Unit Reference Table
| Unit | Symbol | Equivalent in meters |
|---|---|---|
| Nanometers | nm | 1.0e-9 |
| Micrometers | µm | 1.0e-6 |
| Millimeters | mm | 0.001 |
| Centimeters | cm | 0.01 |
| Meters | m | 1 |
| Kilometers | km | 1000 |
| Inches | in | 0.0254 |
| Feet | ft | 0.3048 |
| Yards | yd | 0.9144 |
| Miles | mi | 1609.344 |
Understanding Diameter and Radius
Diameter and radius describe a circle’s basic size. The radius starts at the center and reaches the boundary. The diameter spans two opposite boundary points through the center.
The relationship between both measurements never changes. One diameter contains exactly two radii placed end to end. This simple rule supports fast calculations in every unit.
Unit conversion becomes useful when drawings use different standards. A diameter entered in inches can produce radius in millimeters. The calculator converts through meters before displaying the answer.
Circumference describes the distance around the circular edge. Area measures the surface enclosed inside that edge. Both values depend on the calculated radius and pi.
Batch mode helps when many circular parts require checking. Values can be separated by spaces, commas, or new lines. Each entry receives its own detailed result and measurements.
Precision settings control how many decimal places appear. Scientific notation improves readability for extremely large or tiny values. The original internal calculation keeps greater floating-point precision.
Common uses include pipes, wheels, plates, tanks, and cables. Designers also use radius when creating arcs and rounded corners. Manufacturers often specify products using either diameter or radius.
Always confirm whether a drawing shows radius or diameter. Confusing both measurements can double or halve the required size. Clear unit labels also prevent costly measurement mistakes.
Frequently Asked Questions
1. How do I convert diameter to radius?
Divide the diameter by two. A 20-centimeter diameter has a 10-centimeter radius.
2. How do I convert radius to diameter?
Multiply the radius by two. A 7-inch radius produces a 14-inch diameter.
3. Do radius and diameter use identical units?
Yes. Both are linear measurements, so they normally use the same unit.
4. Can the radius be larger than the diameter?
No. The radius is always exactly half the diameter of the same circle.
5. Can I enter decimal values?
Yes. The calculator accepts whole numbers, decimals, and comma-formatted values.
6. What happens when the value is zero?
A zero diameter gives a zero radius, circumference, and area.
7. Are negative values allowed?
No. Physical radius and diameter measurements cannot be negative.
8. How is circumference calculated?
Circumference equals two times pi times radius, or pi times diameter.
9. How is circle area calculated?
Circle area equals pi multiplied by the square of the radius.